// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2017 Google Inc. All rights reserved.
// http://ceres-solver.org/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// * Redistributions of source code must retain the above copyright notice,
//   this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above copyright notice,
//   this list of conditions and the following disclaimer in the documentation
//   and/or other materials provided with the distribution.
// * Neither the name of Google Inc. nor the names of its contributors may be
//   used to endorse or promote products derived from this software without
//   specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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// Author: yp@photonscore.de (Yury Prokazov)

#ifndef CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_
#define CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_

#include "ceres/concurrent_queue.h"

namespace ceres {
namespace internal {

    // Helper for C++ thread number identification that is similar to
    // omp_get_thread_num() behaviour. This is necessary to support C++
    // threading with a sequential thread id. This is used to access preallocated
    // resources in the parallelized code parts.  The sequence of tokens varies from
    // 0 to num_threads - 1 that can be acquired to identify the thread in a thread
    // pool.
    //
    // If CERES_NO_THREADS is defined, Acquire() always returns 0 and Release()
    // takes no action.
    //
    // If CERES_USE_OPENMP, omp_get_thread_num() is used to Acquire() with no action
    // in Release()
    //
    //
    // Example usage pseudocode:
    //
    // ThreadTokenProvider ttp(N); // allocate N tokens
    // Spawn N threads {
    //    int token = ttp.Acquire(); // get unique token
    //    ...
    //    ... use token to access resources bound to the thread
    //    ...
    //    ttp.Release(token); // return token to the pool
    //  }
    //
    class ThreadTokenProvider {
    public:
        ThreadTokenProvider(int num_threads)
        {
            (void)num_threads;
            for (int i = 0; i < num_threads; i++) {
                pool_.Push(i);
            }
        }

        // Returns the first token from the queue. The acquired value must be
        // given back by Release().
        int Acquire()
        {
            int thread_id;
            CHECK(pool_.Wait(&thread_id));
            return thread_id;
        }

        // Makes previously acquired token available for other threads.
        void Release(int thread_id)
        {
            (void)thread_id;
            pool_.Push(thread_id);
        }

    private:
        // This queue initially holds a sequence from 0..num_threads-1. Every
        // Acquire() call the first number is removed from here. When the token is not
        // needed anymore it shall be given back with corresponding Release()
        // call. This concurrent queue is more expensive than TBB's version, so you
        // should not acquire the thread ID on every for loop iteration.
        ConcurrentQueue<int> pool_;

        ThreadTokenProvider(ThreadTokenProvider&);
        ThreadTokenProvider& operator=(ThreadTokenProvider&);
    };

} // namespace internal
} // namespace ceres

#endif // CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_
